Semiconductor device with low band-to-band tunneling

ABSTRACT

The invention includes a semiconductor device comprising an interlevel dielectric layer over a buried insulator layer over a semiconductor substrate; a source and drain in the interlevel layer; a channel between the source and drain, the channel including a first region having a first bandgap adjacent to a second region having a second bandgap, wherein the first band gap is larger than the second bandgap; and a gate over the channel.

BACKGROUND

The present invention relates to semiconductor devices. Moreparticularly, the invention relates to semiconductor devices having lowband-to-band tunneling.

Band-to-band (BTB) tunneling describes the effect of electrons travelingfrom the valence band through a bandgap to the conduction band of asemiconductor device. As semiconductor devices get smaller, BTBtunneling increases due to higher doping levels, and more recently,through the use of narrow-bandgap materials Conventionalmetal-oxide-semiconductor field-effect transistors (MOSFETs) include achannel region between a source and a drain that is pure silicon. Thismitigates BTB tunneling because silicon has a relatively wide bandgap,but may also limit semiconductor performance. To increase theperformance of a product, some conventional devices employ a channelregion of strained silicon-germanium between the source and drain.However, this approach may increase the total amount of BTB tunneling inthe integrated circuit due to the narrower bandgap of silicon-germanium,especially under compressive strain. The BTB tunneling is particularlyegregious for high-voltage devices such as, for example, 1.8V devicesfor IO applications, which have a channel length much longer than theminimum lithographic capability.

SUMMARY

A first aspect of the invention includes a semiconductor devicecomprising an interlevel dielectric layer on a buried insulator layerover a semiconductor substrate; a source and drain in the interleveldielectric layer; a channel between the source and drain, the channelincluding a first region having a first bandgap adjacent to a secondregion having a second bandgap, wherein the first band gap is largerthan the second bandgap; and a gate over the channel.

A second aspect of the invention includes a semiconductor device with abifurcated bandgap. The semiconductor device comprises a plurality ofsemiconductor structures including at least one long channelsemiconductor structure and at least one short channel semiconductorstructure. The at least one long channel semiconductor structureincludes a first source and a first drain in an interlevel dielectriclayer, a long channel region between the first source and the firstdrain. The long channel region includes a first region having a firstbandgap and a second region having a second bandgap, wherein the firstregion is larger than the second band gap, and a first gate on the longchannel region. The at least one short channel semiconductor structureincludes a second source and a second drain in the interlevel dielectriclayer, a short channel region between the second source and the seconddrain; wherein the first bandgap and the second bandgap bifurcate thebandgap of the semiconductor device.

A third aspect of the invention includes a method of fabricating asemiconductor device. The method comprises forming a buried insulatorlayer over a substrate; depositing a first semiconductor layer having afirst bandgap on the buried insulator layer; depositing a hardmask onthe first semiconductor layer to define at least one long channel regionand at least one short channel region such that the at least one longchannel region is adjacent to the at least one short channel region;epitaxially depositing a second semiconductor layer having a secondbandgap over the first semiconductor layer in the at least one longchannel region and the at least one short channel region, the firstbandgap being larger than the second bandgap; combining the first andsecond layers to create a third semiconductor layer; removing thehardmask to expose the first semiconductor layer remaining under thehardmask; removing a portion of the remaining first semiconductor layerbetween the at least one long channel region and the at least one shortchannel region to substantially separate the at least one long channelregion and the at least one short channel region; and forming a gate oneach of the at least one long channel regions and the at least one shortchannel regions.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various embodiments of the invention, in which:

FIGS. 1-6 show cross-sections of various embodiments of thesemiconductor device.

FIG. 7 shows a flow diagram of a method for fabricating thesemiconductor device.

DETAILED DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention include a high performancesemiconductor device with at least two regions including differentbandgaps, which can reduce band-to-band (BTB) tunneling while retainingmuch of the benefit of performance associated with narrow-bandgapmaterials. A semiconductor device according to embodiments of thepresent disclosure can include a channel between a source and a drain ofthe semiconductor device and a gate over the channel. Devices accordingto the present disclosure can decrease BTB tunneling by providing afirst region of one material having a larger (i.e., wider) bandgapadjacent to the source and/or drain, and a second region of anothermaterial having a smaller (i.e., narrower) bandgap in the center of thechannel to maintain high performance of the semiconductor device. Thatis, a majority of the channel will benefit from strain-induced transportimprovement in the second region of the channel while BTB tunneling willbe reduced due to the first region of the channel having a material witha larger bandgap.

The semiconductor devices described herein may be MOSFETs, or morespecifically, fully depleted silicon on insulator devices (FDSOIs) orFinFETs as are generally known in the art of semiconductormanufacturing. Referring now to FIG. 1 which shows a cross-section of anembodiment of the invention, semiconductor device 10 may include asubstrate 12, a buried insulator layer 14, a channel 20, a source 40, adrain 50, a gate 60, and an interlevel dielectric layer 70. The materialcomposition of substrate 12 may include without limitation: silicon,germanium, silicon germanium, silicon carbide, and those consistingessentially of one or more III-V compound semiconductors having acomposition defined by the formulaAl_(X1)Ga_(X2)In_(X3)As_(Y1)P_(Y2)N_(Y3)Sb_(Y4), where X1, X2, X3, Y1,Y2, Y3, and Y4 represent relative proportions, each greater than orequal to zero and X1+X2+X3+Y1+Y2+Y3+Y4=1 (1 being the total relativemole quantity). Other suitable substrates include II-VI compoundsemiconductors having a composition Zn_(A1)Cd_(A2)Se_(B1)Te_(B2), whereA1, A2, B1, and B2 are relative proportions each greater than or equalto zero and A1+A2+B1+B2=1 (1 being a total mole quantity). Furthermore,a portion or entire semiconductor substrate may be strained.

Buried insulator layer 14 may be disposed on substrate 12. Buriedinsulator layer 14 may include a buried oxide (BOX) layer, a nitride, anoxynitride, or other suitable insulating material(s). In one embodiment,buried insulator layer 14 may include an oxide, such as silicon oxide(SiO₂), hafnium oxide (HfO₂), zirconium oxide (ZrO₂), aluminum oxide(Al₂O₃), titanium oxide (TiO₂), lanthanum oxide (La₂O₃), strontiumtitanate (SrTiO₃), lanthanum aluminate (LaAlO₃), and combinationsthereof.

Source 40 and drain 50 may be disposed on buried insulator layer 14.Source 40 and drain 50 may be formed of any currently known or laterdeveloped semiconductor material(s) or combinations thereof, including,but not limited to: silicon (Si), silicon carbon (SiC), silicongermanium (SiGe), silicon germanium carbon (SiGeC), Ge alloys, galliumarsenic (GaAs), indium arsenic (InAs), indium phosphorus (InP), otheriii-V or ii-VI compound semiconductors, as well as organicsemiconductors. Source 40 and drain 50 may comprise a singlesemiconductor layer, or a multiplicity of semiconductor layers.

Channel 20 may be disposed on buried insulator layer 14 such thatchannel 20 is positioned between source 40 and drain 50. However, it isto be understood that channel 20 may be disposed directly on substrate12 without departing from embodiments of the invention. Channel 20 mayhave a length d1 of approximately 70-150 nanometers (nm) or more (FIGS.1 and 2). It is to be understood that source 40 and drain 50 are notlimited to their respective positions as shown in FIGS. 1-6, but rather,source 40 and drain 50 may be positioned proximate to either side ofchannel 20 such that source 40 and drain 50 are substantially(physically) separated by channel 20. That is, in another embodiment(not shown), source 40 may be positioned where drain 50 is positioned inFIGS. 1-6 and drain 50 may be positioned where source 40 is positionedin FIGS. 1-6. Channel 20 may include at least two different materialsselected from a group consisting of silicon (Si), germanium (Ge), carbon(C), gallium (Ga), arsenic (As), indium (In), aluminum (Al), antimony(Sb), boron (B), lead (Pb), and combinations thereof. Channel 20 mayinclude a first region 22 and a second region 26. First region 22 mayinclude a material having a first bandgap and second region 26 mayinclude another material having a second bandgap. The first bandgap ofthe material in first region 22 may be larger than the second bandgap ofthe material in second region 26. As an example, the larger bandgap maybe 1.0 eV to 1.5 eV, while the smaller bandgap may be 0.6 eV to 1.0 eV.However, it is to be understood that the larger bandgap may exceed 1.5eV, and the smaller bandgap may be less than 0.6 eV without departingfrom embodiments of the invention. For example, first region 22 mayinclude Si and second region 26 may include strained SiGe. In anotherexample, first region 22 may include Ga and second region 26 may includeGa in InAs.

In one embodiment, as shown in FIG. 1, first region 22 may be adjacentto drain 50 and second region 26 may be adjacent to source 40. Firstregion 22 substantially (physically) separates second region 26 fromdrain 50. Band tunneling may be calculated by conventional means, forexample, by using the Wentzel-Kramers-Brillouin (WKB) approximationknown in the art of quantum tunneling calculations. A distance of bandtunneling as used herein, generally refers to a distance of electronstraveling from the valence band through a bandgap to the conduction bandof a semiconductor device, or a length d2 of dimension of first region22. Here, first region 22 may have a length d2 (i.e., extend from drain50 a distance) that is substantially equal to a distance of bandtunneling of drain 50. That is, length d2 of first region 22 may dependon the band tunneling of drain 50. First region 22 may representapproximately 5-10% of length d1 of channel 20. Second region 26 mayrepresent approximately 90-95% of length d1 of channel 20. In anexample, channel 20 can be approximately 140 nm long. In this example,second region 26 may be approximately 130 nm long and first region 22may be 10 nm long.

As shown in FIG. 2, in an alternative embodiment, semiconductor device10 can include a first subregion 22 a and a second subregion 22 b. Thisembodiment may be used where it is not known which end of channel 20that drain 50 is positioned. That is, first subregion 22 a may bepositioned adjacent to source 40 and second subregion 22 b may beadjacent to drain 50. Second subregion 26 can separate first and secondsubregion 22 a, 22 b from each other. First subregion 22 a may have alength d3 that is substantially equal to a distance of band tunneling ofsource 40 when that terminal (source 40) operates as a drain. Secondsubregion 22 b may have a length d2 that is substantially equal to adistance of band tunneling of drain 50. First and second subregions 22a, 22 b may together represent approximately 5-10% of the length d1 ofchannel 20. Second region 26 may represent approximately 90-95% oflength d1 of channel 20. In one example, channel 20 can be approximately140 nm long. In this example, second region 26 may be 120 nm long andfirst and second subregions 22 a, 22 b may each be 10 nm long. Inanother example, in which device 10 is not entirely symmetric, the firstsubregion 22 a may be 5 nm long and second subregion 22 b may be 10 nmlong.

Referring now to both FIGS. 1 and 2, gate 60 may be disposed on channel20 such that gate 60 is over first region 22 and second region 26 asshown in FIG. 1, or over first subregion 22 a, second subregion 22 b,and second region 26 as shown in FIG. 2. Gate 60 may include apolycrystalline silicon (“polysilicon”) electrode and set of spacerswhere desired and/or applicable. However, these elements are omittedfrom FIGS. 1-2 for clarity. Additionally, semiconductor device 10 mayinclude an insulator layer 62 between gate 60 and channel 20. Insulatorlayer 62 may include one or more dielectric materials including but notlimited to: silicon nitride (Si₃N₄), silicon oxide (SiO₂), hafnium oxide(HfO₂), aluminum oxide (AlO₂), nitride, fluorinated oxide, nitratedoxide, other high dielectric constant (>3.9) material, or multiplelayers thereof.

Interlevel dielectric layer 70 may be formed on buried insulator layer14 such that channel 20, source 40, drain 50, and gate 60 aresubstantially surrounded by interlevel dielectric layer 70. Interleveldielectric layer 70 may include one or more dielectric materialsincluding but not limited to: silicon nitride (Si₃N₄), silicon oxide(SiO₂), fluorinated SiO₂ (FSG), hydrogenated silicon oxycarbide (SiCOH),porous SiCOH, boro-phosho-silicate glass (BPS G), silsesquioxanes,carbon (C) doped oxides (i.e., organosilicates) that include atoms ofsilicon (Si), carbon (C), oxygen (O), and/or hydrogen (H), thermosettingpolyarylene ethers, SiLK (a polyarylene ether available from DowChemical Corporation), a spin-on silicon-carbon containing polymermaterial available from JSR Corporation, other low dielectric constant(<3.9) material, or multiple layers thereof. It is to be understood thatinterlevel dielectric layer 70 as described herein may include contacts(not shown) as known in the art of semiconductor manufacturing.

While embodiments of the invention have been described with reference toa FDSOI device, it is to be understood that embodiments of the inventionmay also apply to other semiconductor devices such as a FinFET whichwould operate under much of the same principles as the FDSOI devicedescribed herein but may include a single gate structure and multiplefins. Where semiconductor device 10 is a FinFET, channel 20 may beformed of semiconductor fin (not shown), a portion of which issubstantially surrounded by gate 60 as is well known in the art ofsemiconductor manufacturing. A FDSOI device may have a height of, e.g.,between approximately 4-10 nm and a length of 70-150 nm or more. AFinFET may have a height of between approximately 25-50 nm and a lengthof 70-150 nm or more. That is, a cross-section of a FinFET device maylook substantially similar to a cross-section of the FDSOI device exceptthe channel region of the FDSOI device is relatively shorter in heightand larger in depth.

FIGS. 3 and 4 show another embodiment of the invention. In thisembodiment, semiconductor device 100 may have at least one long channelsemiconductor structure 120 and at least one short channel semiconductorstructure 130. While only a single long channel semiconductor structureand a single short channel semiconductor structure are shown, it is tobe understood that semiconductor device 100 may include multiple shortand long channel semiconductor structures, which are not shown in FIGS.3 and 4 for clarity. Short channel semiconductor devices may be used forlower voltage operations. For those devices it may be impractical toprovide for two separate portions of the channel region, when thechannel length may be not very much larger (15-25 nm) than the typicaltunneling distance of a drain (10 nm). That is, embodiments of thepresent invention provide for a semiconductor device wherein bothdual-material channel devices (for example, long channel semiconductorstructure 120) and single-material devices (for example, short channelsemiconductor structure 130) may coexist.

FIG. 3 shows an embodiment of semiconductor device 100. In thisembodiment, buried insulator layer 114 is disposed on substrate 112.Long channel semiconductor structure 105 and short channel semiconductorstructure 107 are disposed on buried insulator layer 114. Long channelsemiconductor structure 105 may include a first source 140 and a firstdrain 150 substantially separated by a long channel region 120. Firstsource 140 and first drain 150 may be composed of any of the examplematerials discussed herein with respect to source 40 (FIGS. 1, 2) anddrain 50 (FIGS. 1, 2). Long channel region 120 may have a length ofapproximately 70-150 nm. Long channel region 120 may include a firstregion 122 and a second region 126. First region 122 may include amaterial having a first bandgap. Second region 126 may include anothermaterial having a second bandgap such that the first bandgap is largerthan the second bandgap. Long channel region 120 may be composed of anyof the example materials discussed herein with respect to channel 20(FIGS. 1-2). For example, first region 122 may include Si and secondregion 126 may include strained SiGe. In another example, first region122 may include Ga and second region 126 may include Ga in InAs. Firstregion 122 may be adjacent to first drain 150 and second region 126 maybe adjacent to first source 140. In this embodiment, first region 122may have a length (i.e., extend from drain 150 a distance) that issubstantially equal to a distance of band tunneling of first drain 150as described herein. That is, a length of first region 122 may depend onthe band tunneling of first drain 150. As shown in FIG. 3, first region122 substantially separates second region 126 from first drain 150.

Turning to FIG. 4, embodiments of semiconductor device 100 can include afirst subregion 122 a and a second subregion 122 b. First subregion 122a may be adjacent to first source 140 and second subregion 122 b may beadjacent to first drain 150. First subregion 122 a may have a length(i.e., extend from first source 140 a distance) that is substantiallyequal to a distance of band tunneling of source 140 when that terminal(source 140) operates as drain. Second subregion may have a length(i.e., extend from first drain 150 a distance) that is substantiallyequal to a distance of band tunneling of drain 150. In this embodiment,second subregion 126 substantially separates first and second subregion122 a, 122 b from each other.

Referring to FIGS. 3 and 4 together, short channel semiconductorstructure 107 may also be disposed on buried insulator layer 114. Shortchannel semiconductor structure 107 may be substantially (physically)separated from long channel semiconductor structure 105. Short channelsemiconductor structure 107 may include a short channel region 130, asource 142, and a drain 152 on buried insulator layer 114. Short channelregion 130 may have a length of 10-50 nm. Short channel region 132 mayinclude the same material as second region 126 of long channelsemiconductor structure 105. Short channel region 130 can have a bandgapwhich is substantially equal to the bandgap of second region 126 of longchannel semiconductor structure 105 or may in principle be of yetanother material having another bandgap. Short channel region 130 canseparate second source 142 and second drain 152.

Long and short channel semiconductor structures 105, 107 may eachinclude first and second gates 160, 166 respectively. First gate 160 maybe disposed on long channel semiconductor structure 105 such that firstgate 160 is over first region 122 and second region 126 as shown in FIG.3 or over first subregion 122 a, second subregion 122 b, and secondregion 126 as shown in FIG. 4. First gate 160 may include a poly-siliconelectrode and set of spacers, but these elements are omitted from FIGS.3-4 for clarity. Additionally, semiconductor device 100 may include afirst insulator layer 162 positioned between first gate 160 and longchannel semiconductor structure 105.

Second gate 166 may be disposed on short channel semiconductor structure107 such that second gate 166 is over short channel region 132 as shownin FIGS. 3 and 4. Second gate 166 may include a poly-silicon electrodeand set of spacers, but these elements are not shown in FIGS. 3-4 forclarity. Additionally, semiconductor device 100 may include a secondinsulator layer 168 between gate 166 and short channel semiconductorstructure 107. Insulator layers 162, 168 may include any of thematerials discussed previously with reference to insulator layers ofFIGS. 1 and 2. First insulator layer 162 may have a greater thicknessthan a thickness of second insulator layer 168. First insulator layer162 may have a thickness of 2.0-7.0 nm. However, for higher voltageapplications, first insulator layer 162 may have a thickness thatexceeds 7.0 nm. Second insulator layer 168 may have a thickness of 0.6to 1.2 nm. Additionally, an interlevel dielectric layer 170 may beformed on buried insulator layer 114 such that long channelsemiconductor structure 105 and short channel semiconductor structure107 are within interlevel dielectric layer 170. Interlevel dielectriclayer 170 may include any of the materials discussed herein relative tointerlevel dielectric layer 70 of FIGS. 1 and 2. It is to be understoodthat interlevel dielectric layer 170 as described herein may includecontacts (not shown) as known in the art of semiconductor manufacturing.

As discussed previously, embodiments of the invention may apply to aFin-FET device. Where semiconductor device 100 is a FinFET, long channelregion 120 may be in the form of a semiconductor fin (not shown) that issubstantially surrounded by gate 160. Short channel region 130 may be inthe form of a semiconductor fin (not shown), a portion of which can besubstantially surrounded by gate 166.

Semiconductor devices fabricated according to embodiments of theinvention may also contain transistors having entirely different channelmaterial, herein described as a p-type field effect transistor (PFET)and an n-type field effect transistor (NFET). FIGS. 5 and 6 show anotherembodiment of the invention, where a semiconductor device 200 caninclude a PFET 201 adjacent to a NFET 301. In this embodiment, PFET 201may include a first long channel semiconductor structure 205 and a firstshort channel semiconductor structure 207 and NFET 301 may include asecond long channel semiconductor structure 305 and a second shortchannel semiconductor structure 307.

First long channel semiconductor structure 205 and first short channelsemiconductor structure 207 of PFET 201 may be disposed on a buriedinsulator layer 214 over a substrate 212. Second long channelsemiconductor structure 305 and second short channel semiconductorstructure 307 of NFET 301 may also be disposed on buried insulator layer214 over substrate 212. The materials of buried insulator layer 214 andsubstrate 212 may include the same materials discussed with reference toburied insulator layers 14 (FIGS. 1 and 2), 114 (FIGS. 3 and 4).

Referring to FIG. 5, first long channel semiconductor structure 205 caninclude a first source 240 and a first drain 250 substantially separatedby a long channel region 220. First source 240 and first drain 250 maybe composed of any of the materials discussed relative to the previousembodiments. First long channel region 220 may have a length ofapproximately 70-150 nm. First long channel region 220 may be any of thematerials also used in the composition of channel region 20 (FIGS. 1, 2)and long channel region 120 (FIGS. 3, 4). As discussed previouslyherein, first long channel region 220 may include a first region 222 anda second region 226. First region 222 may include a material having afirst bandgap. Second region 226 may include another material having asecond bandgap such that the first bandgap is larger than the secondbandgap. First region 222 may be adjacent to first drain 250 and secondregion 226 may be adjacent to first source 240. Here, first region 222may have a length (i.e., extend from first drain 250 a distance) that issubstantially equal to a distance of band tunneling of first drain 250.That is, a thickness of first region 222 may depend on the bandtunneling of first drain 250. As shown in FIG. 5, first region 222substantially separates second region 226 from first drain 250.

In another embodiment, as shown in FIG. 6, semiconductor device 200 caninclude a first subregion 222 a and a second subregion 222 b asdescribed with respect to first and second subregions 122 a, 122 b(FIGS. 3, 4). First subregion 222 a may be adjacent to first source 240.Second subregion 222 b may be adjacent to first drain 250. Firstsubregion 222 a may have a length (i.e., extend away from first source240 a distance) that is substantially equal to a distance of bandtunneling of first source 240. Second subregion may have a length (i.e.,extend away from first drain 250 a distance) that is substantially equalto a distance of band tunneling of first drain. In this embodiment,second subregion 226 substantially separates first and second subregion222 a, 222 b from each other.

Referring now to FIGS. 5 and 6 together, first short channelsemiconductor structure 207 can include a first short channel region 230composed of a material having a smaller bandgap than the materialcomposition of first region 222 of long channel semiconductor structure220. That is, the material used for first short channel region 230 offirst short channel semiconductor structure 207 may be the same materialused for second region 226 of first long channel semiconductor structure205. In an alternative embodiment, first short channel semiconductorstructure 207 may include a material different second region 226. Shortchannel region 130 may have a length of 15-25 nm. First short channelsemiconductor structure 207 may also include a second source 242 and asecond drain 252. Second source 242 and second drain 252 may besubstantially separated by first short channel region 230.

Second long channel semiconductor structure 305 may include a thirdsource 340 and a third drain 350 substantially separated by a secondlong channel region 320. Third source 340 and third drain 350 mayinclude any of the materials for sources and drains discussed relativeto the previous embodiments. Second long channel region 320 may have alength of approximately 70-150 nm. Second long channel region 320 may beany of the materials also used in the composition of channel region 20(FIGS. 1, 2) and long channel region 120 (FIGS. 3, 4). Second shortchannel semiconductor structure 307 may include a fourth source 342 anda fourth drain 352 substantially separated by a second short channelregion 330. Second short channel region 330 may have a length of 15-25nm. Fourth source 342 and fourth drain 352 may include any of thematerials for sources and drains discussed relative to the previousembodiments. Second short channel region 330 may include any of thematerials for channel regions previously discussed relative to otherembodiments.

Second long channel region 320 of second long channel semiconductorstructure 305 and short channel region 330 of second short channelsemiconductor structure 307 may include a material with a larger bandgapthan the material of first region 222 of first long channelsemiconductor structure 205. That is, the material used for regions 320and 330 may be the same material used for first region 222 of first longchannel semiconductor structure 205. Alternatively, the material usedfor regions 320 and 330 may be different from the material used forfirst region 222. However, while FIGS. 5 and 6 show only first longchannel region 220 of PFET 201 as having a bifurcated bandgap, it is tobe understood that in another embodiment second long channelsemiconductor structure 305 of NFET 301 may also include a second longchannel region 320 with a bifurcated bandgap. Alternatively, NFET 301may include a second long channel region 320 with a bifurcated bandgapin place of first long channel semiconductor structure 205 of PFET 201.

As discussed herein, each semiconductor structure 205, 207, 305, 307 mayalso include a gate 260, 266, 360, 366 respectively. A first gate 260may be disposed on first long channel region 220 such that gate 260 isover first region 222 and second region 226 as shown in FIG. 5.Alternatively, first gate 260 can be positioned over first subregion 222a, second subregion 222 b, and second region 226 as shown in FIG. 6.First gate 260 may include a poly-silicon electrode and set of spacers,which are omitted from FIGS. 5-6 for clarity. Additionally, first longchannel semiconductor structure 205 may include a first insulator layer262 between first gate 260 and first long channel region 220.

A second gate 266 may be disposed on first short channel semiconductorstructure 207 such that gate 266 is over first short channel region 230as shown in FIGS. 5 and 6. Second gate 266 may include a poly-siliconelectrode and set of spacers, which are omitted from FIGS. 5-6 forclarity. Additionally, first short channel semiconductor structure 207may include a second insulator layer 268 between second gate 266 andfirst short channel region 230. Second insulator layer 268 may have alower value of thickness than first insulator layer 262.

A third gate 360 may be disposed on second long channel region 320.Third gate 360 may include a poly-silicon electrode and set of spacersas is well known in the art but not shown in FIGS. 5-6 for clarity.Additionally, second long channel semiconductor structure 305 mayinclude a third insulator layer 362 between third gate 360 and secondlong channel region 320. A fourth gate 366 may be disposed on secondshort channel region 330. Fourth gate 366 may include a poly-siliconelectrode and set of spacers as is well known in the art but not shownin FIGS. 5-6 for clarity. Additionally, second short channelsemiconductor structure 307 may include a fourth insulator layer 368between fourth gate 366 and second short channel region 330. Fourthinsulator layer 368 may be thinner than third insulator layer 362. Inone embodiment, fourth insulator layer 368 may have a thicknesssubstantially equal to second insulator layer 268 and third insulatorlayer 362 may have a thickness substantially equal to first insulatorlayer 262. In another embodiment, insulator layers 262, 268, 362, 368may be of varying thicknesses.

Insulator layers 262, 268, 362, 368 may include any of the materialsdiscussed previously with reference to insulator layers of FIGS. 1 and2. Additionally, an interlevel dielectric layer 270 may be formed onburied insulator layer 214 such that semiconductor structures 205, 207of PFET 201 and semiconductor structures 305, 307 of NFET 301 aresubstantially surrounded by interlevel dielectric layer 270. It is to beunderstood that interlevel dielectric layer 270 as described herein mayinclude contacts (not shown) as known in the art of semiconductormanufacturing.

Referring now to FIG. 7 which shows a method 700 for fabricatingembodiments of the semiconductor devices (i.e., 10 (FIGS. 1, 2), 100(FIGS. 3, 4), 200 (FIGS. 5,6)) described herein. In process P1, a buriedinsulator layer may be formed over a substrate using any now known orlater developed deposition technique, including but not limited to, forexample: chemical vapor deposition (CVD), low-pressure CVD (LPCVD),plasma-enhanced CVD (PECVD), semi-atmosphere CVD (SACVD) high densityplasma CVD (HDPCVD), rapid thermal CVD (RTCVD), ultra-high vacuum CVD(UHVCVD), limited reaction processing CVD (LRPCVD), metalorganic CVD(MOCVD), sputtering deposition, ion beam deposition, electron beamdeposition, laser assisted deposition, thermal oxidation, thermalnitridation, spin-on methods, physical vapor deposition (PVD), atomiclayer deposition (ALD), chemical oxidation, molecular beam epitaxy(MBE), plating, and evaporation. It is to be understood that the use ofthe term “depositing” or “deposited” herein may include any of the abovedeposition techniques. Buried insulator layer and substrate may includeany of the materials previously described with reference to buriedinsulator layers and substrates of the previous embodiments,respectively.

In process P2, a first semiconductor layer having a first bandgap may bedeposited on the buried insulator layer. A hardmask may be deposited onthe first semiconductor layer in process P3. The hardmask may bedeposited to define one or more long channel regions and one or moreshort channel regions such that at least one long channel region isadjacent to at least one short channel region. In process P4, a secondsemiconductor layer having a second bandgap may be epitaxially depositedover the first semiconductor layer in the at least one long channelregion and the at least one short channel region. The second bandgap maybe smaller than the first bandgap. The first material may be silicon andthe second material may be germanium or silicon-germanium.

In process P5, the first and second semiconductor layers may be combinedto create a third semiconductor layer. First and second semiconductorlayers are combined through condensation in an oxidizing ambient or byapplying heat to first and second semiconductor layers. The hardmask canbe removed to expose the first semiconductor layer remaining underneaththe hardmask in process P6. The hardmask may be removed by knownprocesses in the field of semiconductor manufacturing such as etching.Etching may include any now known or later developed techniquesappropriate for the material to be etched including but not limited to,for example: isotropic etching, anisotropic etching, plasma etching,sputter etching, ion beam etching, reactive-ion beam etching andreactive-ion etching (RIE). The placement and removal of the hardmaskallows for self-aligned epitaxial replacement of the end of the channeladjacent to the source and drain as will be discussed.

In process P7, a portion of the remaining first semiconductor layerbetween the at least one long channel region and the at least onechannel region is removed to substantially separate the at least onelong channel region from the at least one short channel region. Firstsemiconductor layer may be removed by example etching processes asdiscussed herein with respect to process P5. During this removalprocess, a portion of first semiconductor layer remains adjacent to afirst side of the third semiconductor layer in the long channel region(as shown in channel region 20 of FIG. 1). That is, the at least onelong channel region may include a first region which may be defined bythe remaining first semiconductor layer. The at least one long channelregion may also include a second region which may be defined by thethird semiconductor layer. The bandgap of the material(s) in the firstregion may be greater than a bandgap of the material(s) in the secondregion. In another embodiment, the portion of the first semiconductorlayer remains adjacent to a first and a second side of the thirdsemiconductor layer in long channel region (shown in channel region 20of FIG. 2). That is, the first region of the at least one long channelregion may include a first subregion and a second subregion that aresubstantially separated by the second region.

In the embodiment described with reference to FIGS. 5-6, the firstsemiconductor layer may be removed in process P7 such that a PFET and aNFET region are formed. That is, the first semiconductor layer may beremoved to define a first region in the first long channel region (tocreate a bifurcated bandgap) and a first short channel region in thePFET region. Additionally, the first semiconductor layer may be removedto define a second long channel region and a second short channel regionin the NFET region. As discussed previously relative to FIGS. 5-6, thelong channel region with a bifurcated bandgap may be in either the PFETregion or the NFET region, or both of the PFET or NFET regions. That is,the removal of the hardmask and the first semiconductor layer may beperformed to customize the location of the portion of the semiconductordevice having the bifurcated bandgap.

In process P8, sources, drains, and gates may be formed. Sources areformed on buried insulator layer. The sources may be formed such that atleast one source is adjacent to a first side of each of the at least onelong channel regions and the at least one short channel regions. Drainsmay also be formed on the buried insulator layer. At least one drain maybe formed adjacent to a second side of each of the at least one longchannel regions and the at least one short channel regions. The sourcesand/or drains may be formed such that each source and drain issubstantially separated by the corresponding channel region. Where theat least one long channel region includes a first region and a secondregion, the drain may be formed adjacent to the first region and thesource may be formed adjacent to the second region. Where the at leastone long channel region includes a first subregion and a secondsubregion, the drain may be formed adjacent to the first subregion andsource may be formed adjacent to the second subregion. As previouslydiscussed, the first subregion and the second subregion may have lengthssubstantially equal to band tunneling of the source and the drainrespectively. The gate is formed on each of the at least one longchannel regions and the at least one short channel regions. The formingof the gate may also may further include depositing a first insulatorlayer on the at least one long channel region prior to forming the gateon the at least one long channel region and depositing a secondinsulator on the at least one short channel region prior to forming thegate on the at least one short channel region. The first and secondinsulator layers may be deposited such that first insulator layer may bethicker than the second insulator layer. Additionally, method 700 mayalso include depositing an interlevel dielectric layer over channels,sources, drains, and gates on buried insulator layer.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

We claim:
 1. A semiconductor device comprising: a dielectric layer on aninsulator layer over a semiconductor substrate; a source and a drain inthe dielectric layer; a channel between the source and drain, thechannel including a first region having a first bandgap adjacent to asecond region having a second bandgap, wherein the first bandgap islarger than the second bandgap; and a gate over the channel.
 2. Thesemiconductor device of claim 1, wherein the first region furthercomprises: a first subregion and a second subregion; wherein the firstsubregion is adjacent to the source and wherein the second subregion isadjacent to the drain.
 3. The semiconductor device of claim 2, whereinthe first subregion extends away from the source a distancesubstantially equal to a distance of band tunneling of the source, andwherein the second subregion extends away from the drain a distancesubstantially equal to a distance of band tunneling of the drain.
 4. Thesemiconductor device of claim 1, wherein the first region is adjacent tothe drain such that the first region substantially separates the secondregion and the drain.
 5. The semiconductor device of claim 1, whereinthe second region is adjacent to the source such that the second regionsubstantially separates the first region and the source.
 6. Thesemiconductor device of claim 1, wherein the first region includessilicon.
 7. The semiconductor device of claim 1, wherein the secondregion includes strained silicon germanium.
 8. The semiconductor deviceof claim 1, wherein the channel includes a semiconductor fin and thegate substantially surrounds at least a portion of the semiconductorfin.
 9. The semiconductor device of claim 3, wherein the first regionfurther comprises a first subregion and a second subregion.
 10. Thesemiconductor device of claim 9, wherein the second region substantiallyseparates the first subregion and the second subregion.
 11. Thesemiconductor device of claim 9, wherein the first subregion extendsaway from the source a distance substantially equal to a distance ofband tunneling of the source, and wherein the second subregion extendsaway from the drain a distance substantially equal to a distance of bandtunneling of the drain.